Contact and adsorbent granules

ABSTRACT

The present invention relates to a filtering unit at least partially filled with particles agglomerated from fine-particle iron oxide and/or iron oxyhydroxide by producing an aqueous suspension of fine-particle iron oxides and/or iron oxyhydroxides having a BET surface area of 50 to 500 m 2 /g, and removing the water and dissolved constituents by a set of washing drying and filtering steps and processes of using the particles.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to particles, pellets or granulesof fine-particle or nanoparticle iron oxides and/or iron oxyhydroxideshaving a large specific surface area (50 to 500 m²/g according to BET),and processes for their production. These pellets have high mechanicalresistance and can be used as a contact, adsorbent, or catalyst for thecatalysis of chemical reactions, for the treatment of fluid media likeliquids and/or for gas, specifically the removal of impurities.

[0002] Contact and adsorbent granules, including those based on ironoxides and/or iron oxyhydroxides, have already been described. They arepredominantly used in continuous processes. They are conventionallyfound in tower or column-type units through which the medium to betreated flows, and the chemical or physical reaction or adsorptionprocesses take place at the outer and inner surface of the granules.Powdered materials cannot be used for this purpose because they compactin the direction of flow of the medium, thereby increasing the flowresistance until the unit becomes blocked. If a unit is cleaned byback-flushing (see below), large amounts of the powder are dischargedand lost or cause an unacceptable contamination of the waste water.

[0003] The flowing media also exert forces on the granules, however,which can lead to abrasion and/or movement through to violent agitationof the granules. Consequently the granules collide, leading toundesirable abrasion. This results in loss of contact or adsorbentmaterial and contamination of the medium to be treated.

[0004] Adsorbents/catalysts containing iron oxides and hydroxides canadvantageously be used e.g. in the area of water purification or gaspurification. In water purification this agent is used in horizontal- orvertical-flow filters or adsorber columns or added to the water to betreated in order to remove dissolved, suspended or emulsified organic orinorganic phosphorus, arsenic, antimony, sulfur, selenium, tellurium,beryllium, cyano and heavy metal compounds from, for example, drinkingwater, process water, industrial and municipal waste water, mineral,holy and medicinal water as well as garden pond and agricultural water.It can also be used in so-called reactive walls to separate the citedcontaminants from ground water and seepage water aquifers fromcontaminated sites (waste disposal sites).

[0005] In gas purification the agent is used in adsorbers for bindingundesirable components such as hydrogen sulfide, mercaptans and hydrogencyanide, as well as other phosphorus, arsenic, antimony, sulfur,selenium, tellurium, cyano and heavy metal compounds in waste gases.Gases such as HF, HCl, H₂S, SO_(x), NO_(x) can also be adsorbed.

[0006] The removal of phosphorus, arsenic, antimony, selenium,tellurium, cyano and heavy metal compounds from waste oils and othercontaminated organic solvents is also possible.

[0007] Contact and adsorbent granules based on iron oxides and/or ironoxyhydroxides are also used for the catalysis of chemical reactions inthe gas phase or in the liquid phase.

[0008] Various methods of removing trace constituents and contaminantsfrom aqueous systems with the aid of adsorbents are also known.

[0009] For example, DE-A 3 120 891 describes a process in which afiltration is performed using activated alumina with a grain size of 1to 3 mm for the separation principally of phosphates from surface water.

[0010] DE-A 3 800 873 describes an adsorbent based on porous materialssuch as e.g. hydrophobed chalk with a fine to medium grain size toremove contaminants from water.

[0011] DE-A 3 703 169 discloses a process for the production of agranulated filter medium to treat natural water. The adsorbent isproduced by granulating an aqueous suspension of kaolin with addition ofpowdered dolomite in a fluidised bed. The granules are then baked at 900to 950° C.

[0012] A process for the production and use of highly reactive reagentsfor waste gas and waste water purification is known from DE-A 40 34 417.Mixtures consisting of Ca(OH)₂ with additions of clays, stone dust,entrained dust and fly ashes, made porous and having a surface area ofapprox. 200 m²/g, are described here.

[0013] The cited processes have the disadvantage that the componentresponsible in each case for the selective adsorption of constituents ofthe media to be cleaned, in other words the actual adsorbent, must besupplemented with large quantities of additives to enable it to beshaped into granules. This significantly reduces the binding capacityfor the water contaminants to be removed. Moreover, subsequentreprocessing or reuse of the material is problematic since the bindersubstances first have to be separated out.

[0014] DE-A 4 214 487 describes a process and a reactor for the removalof impurities from water. The medium flows horizontally through afunnel-shaped reactor, in which finely divided iron hydroxide inflocculent form is used as a sorption agent for water impurities. Thedisadvantage of this process lies in the use of the iron hydroxide inflocculent form, which means that because there is little difference indensity between water and iron hydroxide, a reactor of this type can beoperated at only very low flow rates and there is a risk of the sorptionagent, which is possibly already loaded with contaminants, beingdischarged from the reactor along with the water.

[0015] JP-A 55 132 633 describes granulated red mud, a by-product ofaluminium production, as an adsorbent for arsenic. This consists ofFe₂O₃, Al₂O₃ and SiO₂. No mention is made of the stability of thegranules or of the granulation process. A further disadvantage of thisadsorbent is the lack of consistency in the composition of the product,its unreliable availability and the possible contamination of thedrinking water with aluminium. Since aluminium is suspected ofencouraging the development of Alzheimer's Disease, contamination withthis substance in particular is to be avoided.

[0016] DE-A 19 826 186 describes a process for the production of anadsorbent containing iron hydroxide. An aqueous polymer dispersion isincorporated into iron hydroxide in water-dispersible form. This mixtureis then either dried until it reaches a solid state and the solidmaterial then comminuted mechanically to the desired shape and/or sizeor the mixture is shaped, optionally after a preliminary drying stage,and a final drying stage then performed, during which a solid state isachieved. In this way a material is obtained in which the iron hydroxideis firmly embedded in the polymer and which is said to display a highbinding capacity for the contaminants conventionally contained in wastewaters or waste gases.

[0017] The disadvantage of this process lies in the use of organicbinders, which further contaminate the water to be treated due toleaching and/or abrasion of organic substances. Furthermore, thestability of the adsorbent composite is not guaranteed in extended use.Bacteria and other microorganisms can also serve as a nutrient mediumfor an organic binder, presenting a risk that microorganisms maypopulate the contact and thereby contaminate the medium.

[0018] The presence of foreign auxiliary substances, which are requiredfor the manufacture of the adsorbents, during reprocessing, recycling orreuse of used adsorbents is disadvantageous in principle because thereuse of pure substances is less problematic than is the case with mixedsubstances. For example, polymeric binders are disadvantageous when ironoxide-based adsorption materials are reused as pigments for concretecoloration because these binders inhibit dispersion of the pigment inliquid concrete.

[0019] DE-A 4 320 003 describes a process for the removal of dissolvedarsenic from ground water with the aid of colloidal or granulated ironhydroxide. Where fine, suspended iron(III) hydroxide products are used,it is recommended here that the iron hydroxide suspension be placed infixed-bed filters filled with granular material or other supports havinga high external or internal porosity. This process likewise has thedisadvantage that, relative to the adsorbent “substrate+iron hydroxide”,only low specific loading capacities are achievable. Furthermore, thereis only a weak bond between substrate and iron hydroxide, which meansthat there is a risk of iron hydroxide or iron arsenate being dischargedduring subsequent treatment with arsenic-containing water. Thispublication also cites the use of granulated iron hydroxide as anadsorption material for a fixed-bed reactor. The granulated ironhydroxide is produced by freeze conditioning (freeze drying) of ironhydroxide obtained by neutralisation of acid iron(III) salt solutions attemperatures of below minus 5° C. This production process is extremelyenergy-intensive and leads to heavily salt-contaminated waste waters.Moreover, as a result of this production process only very smallgranules with low mechanical resistance are obtained. When used in afixed-bed reactor, this means that the size spectrum is significantlyreduced by mechanical abrasion of the particles during operation, whichin turn results in finely dispersed particles of contaminated oruncontaminated adsorption agent being discharged from the reactor. Afurther disadvantage of these granules lies in the fact that theadsorption capacity in respect of arsenic compounds is reducedconsiderably if the granules lose water, by being stored dry forextended periods for example.

[0020] Adsorbent/binder systems obtained by removing a sufficientlylarge amount of water from a mixture of (a) a crosslinkable binderconsisting of colloidal metal or non-metal oxides, (b) oxidic adsorbentssuch as metal oxides and (c) an acid such that components (a) and (b)crosslink to form an adsorbent/binder system, are known from U.S. Pat.No. 5,948,726. According to the disclosure, colloidal alumina oraluminium oxide is used as binder.

[0021] The disadvantage of these compositions lies in the need to useacid in their production (column 9, line 4) and in the fact that theyare not pure but heterogeneous substances, which is undesirable both forthe production, regeneration, removal and permanent disposal of suchadsorbents, e.g. on a waste disposal site. The scope of disclosure ofthis publication is also intended to include adsorbents that aresuitable for the adsorption of arsenic; specific examples are notprovided, however. Aluminium oxide is known to be significantly inferiorto iron oxides in regard to force of adsorption for arsenic.

[0022] Continuous adsorbers, which are commonly grouped together inparallel for operation, are preferably used for water treatment. To freedrinking water from organic impurities, for example, such adsorbers arefilled with activated carbon. At peak consumption times, the availableadsorbers are then operated in parallel to prevent the flow rate fromrising above the upper limit permitted by the particular arrangement. Attimes of lower water consumption, individual adsorbers are taken out ofoperation and can be serviced, for example, whereby the adsorptionmaterial is subjected to special loads, as described in greater detailbelow.

[0023] The use of granules, which can be produced by compacting e.g.powdered iron oxide using high linear forces, has also already beenconsidered. Such granules have already been described as a means ofhomogeneously colouring liquid concrete. The use of high linear forcesfor compacting is extremely expensive and energy-intensive, and thestability of the compacted materials is inadequate for extended use inadsorbers.

[0024] The use of such materials in adsorbers, for example, particularlycontinuous models, for water purification is therefore of only limitedinterest. During maintenance or cleaning of adsorber plants byback-flushing in particular (see below), such granules lose largeamounts of substance due to the associated agitation. The abradedmaterial renders the waste water from back-flushing extremely turbid.This is unacceptable for a number of reasons: firstly, adsorptionmaterial, which is heavily laden with impurities and therefore toxicafter extended use, is lost. Secondly, the stream of waste water isladen with abraded material, which can sediment, damaging piping systemsand ultimately subjecting the waste treatment plant to undesirablephysical and toxicological stresses, to name but a few reasons.Preferably the abrasion should be below 20% by weight, more preferablybelow 15% by weight, 10% by weight or most preferably below 5% by weightaccording to the method described in the examples of the presentinvention.

[0025] An object of the present invention was therefore to provide acontact or an adsorbent/catalyst based on iron-oxygen compounds inpellet form, exhibiting high mechanical resistance in conjunction with agood binding capacity for contaminants contained in liquids and gaseswithout the need to use organic binders or inorganic foreign binders toachieve adequate mechanical resistance, and plants operated with suchmedia. This object is achieved by the contacts or adsorbents/catalystsaccording to the invention, their preparation, their use and the unitsfilled therewith.

SUMMARY OF THE INVENTION

[0026] The invention relates to a unit suitable for the through-flow ofa fluid medium at least partially filled with particles agglomeratedfrom fine-particle iron oxide and/or iron oxyhydroxide, wherein thefine-particle iron oxide and/or iron oxyhydroxide displays a particlesize of up to 500 nm and a BET surface area of 50 to 500 m²/g.

[0027] The invention also relates to a process for the production ofparticles from fine-particle iron oxide and/or iron oxyhydroxidecomprising the steps of producing an aqueous suspension of fine-particleiron oxides and/or iron oxyhydroxides having a BET surface area of 50 to500 m²/g, and removing the water and dissolved constituents by either I)a) first removing only the water from the suspension, b) introducing theresidue thus obtained in water, c) filtering the material obtained, d)washing the residue, and e) either e1) completely dehydrating the filtercake obtained as residue and comminuting the material thus obtained tothe desired shape and/or size or e2) partially dehydrating thefiltercake to obtain a paste, shaping the paste and subsequentlyadditionally drying the paste until a pellet is obtained, or II) a)filtering the suspension, b) washing the residue, c) either c1)completely dehydrating the filter cake obtained as residue in the formof a solid to semisolid paste and then comminuting the material thusobtained to the desired shape and/or size or c2) partially dehydratingthe filtercake to obtain a paste, shaping the paste, and subsequentlyadditionally drying the paste until a pellet is obtained.

DETAILED DESCRIPTION OF THE INVENTION

[0028] To prepare the granules according to the invention, an aqueoussuspension of fine-particle iron oxyhydroxides and/or iron oxides isfirst produced according to the prior art. The water and constituentsdissolved within it can be removed from this in two different ways:

[0029] Method 1:

[0030] For applications in which lower demands are made of themechanical strength of the granules/contacts, only the water is removedinitially, e.g. by evaporation. A residue is obtained which in additionto the fine-particle iron oxide and/or hydroxide also contains theentire salt content. This residue is redispersed in water after beingdried, for which purpose only relatively little shear force needs to beapplied. This suspension is then filtered and the residue washed untilit is substantially free from salts. The filter cake obtained as residueis a solid to semisolid paste which generally has a water content ofbetween 10 and 90 wt. %.

[0031] This can then be completely or partially dehydrated, and thematerial thus obtained can then be comminuted to the desired shapeand/or size. Alternatively the paste or filter cake, optionally afterpredrying to achieve a sufficiently solid state, can undergo shapingfollowed by (additional) drying until a pellet state is achieved. Thesubsequent application of the granules determines the preferredprocedure to be followed for their production, which can be determinedby the person skilled in the art in the particular field of applicationby means of simple preliminary orienting experiments. Both the directlydried filter cake and the dried shaped bodies can then be used ascontact or adsorbent.

[0032] Method 2:

[0033] For applications in which higher demands are made of themechanical strength of the granules/contacts, the suspension is filteredand the residue washed until it is substantially free from salts. Thefilter cake obtained as residue is a solid to semisolid paste. This canthen be completely or partially dehydrated, and the material thusobtained can then be comminuted to the desired shape and/or size.Alternatively the paste or filter cake, optionally after predrying toachieve a sufficiently solid state, can undergo shaping followed by(additional) drying until a pellet state is achieved. The subsequentapplication of the granules determines the preferred procedure to befollowed for their production, which can be determined by the personskilled in the art in the particular field of application by means ofsimple preliminary orienting experiments. Both the directly dried filtercake and the dried shaped bodies can then be used as contact oradsorbent.

[0034] Although the products obtained according to method 1 are lessmechanically resistant, filtration can be performed more easily andquickly. The fine-particle pigments isolated in this way can moreover beincorporated very easily into paints and polymers, for example, becauseconsiderably less shear force is required than is needed to incorporatethe fine-particle pigments obtained according to method 2.

[0035] The fine-particle iron oxide and/or iron oxyhydroxide used has aparticle size of up to 500 nm, preferably up to 100 nm, particularlypreferably 4 to 50 nm, and a BET surface area of 50 to 500 m²/g,preferably 80 to 200 m²/g.

[0036] The primary particle size was determined by measurement fromscanning electron micrographs, e.g. at a magnification of 60000:1(instrument: XL30 ESEM FEG, Philips). If the primary particles areneedle-shaped, as in the α-FeOOH phase for example, the needle width canbe given as a measurement for the particle size. Needle widths of up to100 nm, but mainly between 4 and 50 nm, are observed in the case ofnanoparticle α-FeOOH particles. α-FeOOH primary particles conventionallyhave a length:width ratio of 5:1 to 50:1, typically of 5:1 to 20:1. Thelength:width ratio of the needle shapes can be varied, however, bydoping or by special reaction processes. If the primary particles areisometric, as in the α-Fe₂O₃, γ-Fe₂O₃, Fe₃O₄ phases for example, theparticle diameters can quite easily also be below 20 nm.

[0037] By mixing nanoparticle iron oxides or iron (oxy)hydroxides withpigments and/or Fe(OH)₃, the presence of the cited pigment or nucleusparticles in their known particle morphology, held or glued together bythe nanoparticle nucleus particles or the amorphous Fe(OH)₃ polymer, canbe detected on the scanning electron micrographs.

[0038] Products obtainable by methods 1) and 2) can then be comminutedfurther, for example by rough grinding or grinding. However, since theproducts reduce in size on first coming into contact with water, forexample when a freshly charged adsorber unit is first filled with water,this will generally be unnecessary.

[0039] Granulation of a semi-wet paste has proven effective as anothermethod of producing granules. Here pellets or strands are formed from asemi-wet paste, e.g. using a simple perforated metal sheet, a roll pressor an extruder, and either dried immediately or additionally shaped intoa spherical or granular form by means of a spheroniser. The still wetspherules or granules can subsequently be dried to any moisture contentwhatsoever. A residual moisture content of <50% is recommended toprevent the granules from agglomerating. A spherical shape of this typecan be advantageous for use in fixed-bed adsorbers due to the improvedpacking in the adsorber vessel that is obtained in comparison withrough-ground granules or pellets in strand form.

[0040] The filtration performance of the suspensions can generally beimproved by the use of conventional filtration-improving measures, suchas are described for example in Solid-Liquid Filtration and SeparationTechnology, A. Rushton, A. S. Ward, R. G. Holdich, 2nd edition 2000,Wiley-VCH, Weinheim, and in Handbuch der IndustriellenFest/Flüssig-Filtration, H. Gasper, D. Öchsle, E. Pongratz, 2nd edition2000, Wiley-VCH Weinheim. Coagulants can thus be added to thesuspensions, for example.

[0041] Iron carbonates can also be used in addition to or in place ofthe iron oxyhydroxides.

[0042] The products according to the invention can undergo drying inair, and/or in vacuo, and/or in a drying oven and/or on belt dryers orby spray drying, preferably at temperatures from −25 to 250° C.,particularly preferably at 60 to 120° C.

[0043] The products according to the invention preferably have aresidual water content of less than 20 wt. %.

[0044] It was found that the pellets or granules obtained in this wayhave a high binding capacity for contaminants contained in water,liquids or gases and they additionally have an adequately highresistance to flowing media in terms of mechanical or hydraulicstressing.

[0045] It is particularly surprising that during drying, fine-particleiron oxyhydroxides or iron oxides having large specific surface areassolidify into very hard agglomerates, which without the addition ofbinders have a high mechanical abrasion resistance and high hydraulicresistance to contact with flowing water, and which have a high bindingcapacity for the contaminants and trace constituents contained in thewater.

[0046] Transparent iron oxyhydroxide pigments, for example, having anaverage particle size of less than 0.1 μm and specific surface areas ofgreater than 80 m², are suitable for the use according to the inventionof fine-particle iron oxyhydroxides. Correspondingly fine-particle ironoxide pigments, preferably haematite, magnetite or maghemite, can alsobe used, however.

[0047] The production of yellow fine-particle iron oxyhydroxide pigments(e.g. goethite) in the acid or alkaline pH range, known as acid oralkaline nuclei, is known. The production of other fine-particle ironoxide or iron oxyhydroxide pigments is also known. Such pigments cancontain structures based on α, β, γ, δ, δ′, ε phases and/or Fe(OH)₂ andmixed and intermediate phases thereof. Fine-particle yellow ironoxyhydroxides can be calcined to fine-particle red iron oxides.

[0048] The production of transparent iron oxides and iron oxyhydroxidesis known e.g. according to DE-A 2 603 050 from BIOS 1144, p. 29 to 33 orfrom FIAT 814, p. 1 to 26.

[0049] Fine-particle yellow iron oxyhydroxide pigments are generallysynthesized by precipitating iron(II) hydroxides or carbonates fromcorresponding iron(II) salt solutions such as e.g. FeSO₄, FeCl₂ in pureform or as pickling solutions in the acid or alkaline pH range, followedby oxidation to iron(III) oxyhydroxides (see inter alia G. Buxbaum,Industrial Inorganic Pigments, VCH Weinheim, 2nd edition, 1998, p.231ff). Oxidation of the divalent to the trivalent iron is preferablyperformed with air, whereby intensive aeration is advantageous.Oxidation with H₂O₂ also leads to fine-particle iron oxyhydroxides. Thetemperature chosen for precipitation and oxidation should be as low aspossible in order to obtain very fine-particle yellow pigments. It ispreferably between 15° C. and 45° C. NaOH is preferably used as alkalineprecipitant. Other precipitants, such as KOH, Na₂CO₃, K₂CO₃, CaO,Ca(OH)₂, CaCO₃, NH₃, NH₄OH, MgO and/or MgCO₃, can also be used, however.

[0050] By choosing suitable precipitation and oxidation conditions,nanoparticle α, β, γ, δ phases and mixed phases of iron oxyhydroxidesdisplaying a large specific surface area can be prepared, such that thenanoparticles agglomerate in the dry state and possess a high resistanceto mechanical and fluid-mechanical abrasion in comminuted form.

[0051] Production of fine-particle iron oxyhydroxides by simultaneousrapid treatment of iron(II) salt solutions with NaOH and air has provento be particularly beneficial in practice because this production methodleads to particularly fine-particle iron (oxy)hydroxides and hence to agreater stability of the finished product in addition to a higher forceof adsorption.

[0052] To steer the precipitated pigments in the direction of theextremely fine-particle character that is required, the precipitations,e.g. of yellow α-FeOOH as described in patents U.S. Pat. No. 2,558,303and U.S. Pat. No. 2,558,304, are performed in the alkaline pH range withalkali carbonates as precipitants, and modifiers such as SiO₂, zinc,aluminium or magnesium salts, hydroxycarbonic acids, phosphates andmetaphosphates are generally added. Products produced in this way aredescribed in U.S. Pat. No. 2,558,302. Such nucleus modifiers do notinterfere with the subsequent reprocessing, recycling or any other useof the adsorbents according to the invention. In the case ofprecipitation processes in an aqueous medium, it is known thatprecipitations in an alkaline environment lead to less solidlyagglomerated powders than those in an acid environment.

[0053] One of the advantages of nucleus modifiers, however, is that anadequate fine-particle character can be obtained even at elevatedreaction temperatures.

[0054] DE-A 4 235 945 reports on the production of fine-particle ironoxides using a precipitation method in the acid pH range and withoutmodifiers.

[0055] DE-A 4 434 669 describes a process by which highly transparentyellow, chemically pure iron oxide pigments can be produced by secondarytreatment thereof with sodium hydroxide solution.

[0056] DE-A 4 434 972 reports on highly transparent, yellow iron oxidepigments in the α-FeOOH modification having a specific surface area ofover 100 m²/g and high temperature resistance.

[0057] DE-A 4 434 973 describes highly transparent yellow iron oxidepigments, which are produced by means of the process steps of nuclearprecipitation in the acid pH range, nuclear oxidation, nuclearmaturation and pigment formulation.

[0058] Red, transparent iron oxide pigments obtained by calcining fromyellow, transparent iron oxide pigments are known from DE-A 4 434 668and DE-A 4 235 946.

[0059] By preparing diverse phases of iron oxyhydroxides in pure form orin any mixture from iron(II) salt solutions using the knownprecipitation and oxidation reactions, separating the resultant ironoxyhydroxides out of the suspension, optionally after a secondarytreatment, by filtering the salt solution and washing them until theyare largely free from salts, preferably down to a residual conductivityof <5 mS/cm, then drying the solid or semisolid filter cake just as itis or optionally after mechanical shaping until it achieves a solidstate, a mechanically highly resistant material displaying a highbinding capacity for the contaminants conventionally contained in wastewaters or waste gases is obtained.

[0060] Drying is conveniently performed at temperatures of up to 250° C.The material can also be vacuum or freeze dried. The particle size ofthe material can be freely selected but is preferably between 0.2 and 40mm, particularly preferably between 0.2 and 20 mm. This can be achievedby shaping the semisolid, pasty filter cake mechanically, before drying,in a granulation or pelletising plant or in an extruder to form shapedbodies whose size is in the range between 0.2 and 20 mm, with subsequentdrying in the air, on a belt dryer or in a drying oven, and/or bymechanical comminution to the desired particle size after drying.

[0061] The products described, the process for their production andtheir use represent an improvement over the prior art. In contrast tothose based on coarse-particle iron oxyhydroxides and/or oxides, thegranules according to the invention based on fine-particle ironoxyhydroxides and/or oxides can be subjected to much higher stresses andtherefore display a much greater abrasion resistance to mechanical andhydraulic stressing. They can be used directly as such. When used inadsorber plants for water purification, for example, there is no needeven for comminution or rough grinding of the crude dry substanceinitially obtained from filter cakes or extruders, since the coarsepellets break down independently on contact with water. This results ina random particle-size distribution, but no particles of such a sizethat they are discharged from the adsorber to any significant extent bythe flowing medium.

[0062] There is absolutely no need for a separate granulation process,such as would be necessary when using conventional iron oxyhydroxides inthe form of (flowable) powders, either with the aid of foreign bindersor using extremely high linear forces during compacting.

[0063] According to the invention, the suspensions of fine-particle ironoxyhydroxides or iron oxides can also be supplemented with conventionalpowdered iron oxyhydroxides or iron oxides. The quantities in each caseare determined by the properties of these powdered iron oxyhydroxides oriron oxides and by the requirements of the product according to theinvention in terms of its mechanical stability and abrasion resistance.Although the addition of powdered pigments will generally reduce themechanical strength of the products according to the invention,filtration of the fine-particle suspensions is made easier. The personskilled in the art and practising in the particular field of applicationwill be able to determine the optimum mixing ratio for the intendedapplication by means of a few orienting experiments.

[0064] The nanoparticle nuclei are conveniently produced in an excess ofsodium hydroxide solution.

[0065] A quantity of Fe₂(SO₄)₃ corresponding to the NaOH excess can alsobe added to the suspensions of the alkaline fine-particle nuclei. Thismeasure considerably improves the filterability of the suspension. Theinitially amorphous Fe(OH)₃ produced matures over time, to the α-FeOOHphase, for example. This ensures that the sodium hydroxide solution usedin excess to produce the alkaline nucleus is completely used up. Thematerial thus obtained also displays large specific surface areas. Justlike the iron oxyhydroxides described above, the material is extremelysuitable for use in adsorbers since it possesses a high resistance tomechanical loading in addition to a high adsorption capacity.

[0066] The granules according to the invention are particularlypreferably used in the cleaning of liquids, especially for the removalof heavy metals. A preferred application in this industrial field is thedecontamination of water, particularly of drinking water. Particularattention has recently been paid to the removal of arsenic from drinkingwater. The granules according to the invention are extremely suitablefor this purpose, since levels that not only meet but actually fallbelow even the lowest limiting values set by the US authority the EPAcan be achieved using the granules according to the invention.

[0067] To this end the granules can be used in conventional adsorberunits, such as are already used with a charge of activated carbon, forexample, to remove other types of contaminants. Batchwise operation, incisterns or similar containers for example, optionally fitted withagitators, is also possible. However, use in continuous plants such ascontinuous-flow adsorbers is preferred.

[0068] Since untreated water to be processed into drinking waterconventionally also contains organic impurities such as algae andsimilar organisms, the surface of adsorbents, especially the outersurface of granular adsorbents, becomes coated during use with mostlyslimy deposits, which impede or even prevent the inflow of water andhence the adsorption of constituents to be removed. For this reasonadsorber units are periodically back-flushed with water, a process whichis preferably performed at times of low water consumption (see above) onindividual units that have been taken out of service. The adsorbent iswhirled up and the associated mechanical stress to which the surface issubjected causes the undesirable coating to be removed and dischargedagainst the direction of flow during active operation. The wash water isconventionally sent to a sewage treatment plant. The adsorbentsaccording to the invention have proven to be particularly effective inthis process, since their high strength enables them to be cleanedquickly without suffering any significant losses of adsorption materialand without the back-flush water sent for waste treatment being rich indischarged adsorption material, which is possibly already highlycontaminated with heavy metals.

[0069] Since the granules according to the invention are free fromforeign binders, the material is comparatively easy to dispose of afteruse. For instance, the adsorbed arsenic can be removed by thermal orchemical means in special units, for example, resulting in an iron oxidepigment as a pure substance which can either be recycled for use in thesame application or supplied for conventional pigment applications.Depending on the application and legal regulations, the content of theadsorber can also be used without prior removal of the heavy metals, forexample as a pigment for colouring durable construction materials suchas concrete, since the heavy metals removed from the drinking water arepermanently immobilised in this way and taken out of the hydrologicalcycle.

[0070] The invention therefore also provides water treatment plants orwaterworks in which units filled with the granules according to theinvention are operated, and processes for the decontamination of waterby means of such units, as well as such units themselves.

[0071] For many applications, particularly those in which a maximummechanical strength is not required of the granules, the addition ofpowdered pigments during production of the granules according to theinvention is a preferred embodiment.

[0072] Thus, for example, up to 40 wt. % of commercial goethite(Bayferrox 920, Bayer A G, Leverkusen D E) can be added to a nucleussuspension according to example 2 of the present application if thegranules obtained according to the invention are to be used for theremoval of arsenic from drinking water in adsorbers with a through-flowof water.

[0073] The BET specific surface area of the products according to theinvention is determined by the carrier gas process (He:N₂=90:10) usingthe single-point method, according to DIN 66131 (1993). The sample isbaked for 1 h at 140° C. in a stream of dry nitrogen before measurement.

[0074] In order to measure the adsorption of arsenic(III) andarsenic(V), 3 liters of an aqueous solution of NaAsO₂ or Na₂HAsO₄, eachwith the specified concentration of approx. 2-3 mg/l arsenic, aretreated with 3 g of the sample to be tested in a 5 liter PE flask for aspecific period and the flask moved on rotating rollers. The adsorptionrate of As ions on iron hydroxide over this specific period, e.g. onehour, is stated as mg(As^(3+/5+))/g(FeOOH).h, calculated from thebalance of the As^(3+/5+) ions remaining in solution.

[0075] The adsorption of Sb³⁺, Sb⁵⁺, Hg²⁺, Pb²⁺, Cr⁶⁺ or Cd²⁺ ions ismeasured in the same way, whereby the desired concentrations areestablished by dissolving appropriate amounts of Sb₂O₃, KSb(OH)₆, PbCl₂,NaCrO₄ or CdCl₂ in H₂O and adjusting the pH value to 7-9.

[0076] The As, Sb, Cd, Cr, Hg or Pb contents of the contaminated ironoxyhydroxide or of the solutions are determined using mass spectrometry(ICP-MS) according to DIN 38406-29 (1999) or by optical emissionspectroscopy (ICP-OES) according to EN-ISO 11885 (1998), withinductively coupled plasma as excitation agent in each case.

[0077] The mechanical and hydraulic abrasion resistance was assessedusing the following method: 150 ml of demineralised water were added to10 g of the granules to be tested, having particle sizes >0.1 mm, in a500 ml Erlenmeyer flask, which was rotated on a LabShaker shakingmachine (Kühner model from Braun) for a period of 30 minutes at 250 rpm.The >0.1 mm fraction was then isolated from the suspension using ascreen, dried and weighed. The weight ratio between the amount weighedout and the amount weighed in determines the abrasion value in %.

[0078] The invention is described in greater detail in the following bymeans of examples. The examples are intended to illustrate the processand do not constitute a limitation.

EXAMPLES Example 1

[0079] 237 l of an aqueous iron sulfate solution with a concentration of150 g/l FeSO₄ were prepared at 24° C. 113 l of an aqueous NaOH solution(227 g/l) were then quickly added and the light blue suspension thenoxidised with 40 l of air per hour and per mol of iron for 1.5 hours.

[0080] The yellow suspension thus obtained was filtered out through afilter press and the solid washed until the residual filtrateconductivity was 1 mS/cm. The filter cake was in the form of aspreadable and kneadable paste, which was dried on metal sheets in acirculating air drying oven at 75° C. until the residual moisturecontent was 3 wt. %. The dried material was then roughly ground toproduce particle sizes of between 0.5 and 2 mm. The hard pellets thusobtained were then placed directly in an adsorber tank.

[0081] The product consisted of 100% α-FeOOH with an extremelyshort-needled habit, whereby the needles were congregated to form solidmacroscopic agglomerates. Using a scanning electron micrograph e.g. at amagnification of 60000:1, the needle widths were measured at between 15and 35 nm, the needle lengths between 150 and 350 nm. The needles wereextremely agglomerated.

[0082] The BET specific surface area was 122 m²/g. The adsorption ratefor NaAsO₂ with an original concentration of 2.3 mg (As³⁺)/l was 2.14mg(As³⁺)/g(FeOOH).h, for Na₂HAsO₄ with an original concentration of 2.7mg (As⁵⁺)/l it was 2.29 mg(As⁵⁺)/g(FeOOH).h.

Example 2

[0083] 800 l of an aqueous iron sulfate solution with a concentration of150 g/l FeSO₄ were prepared at 29° C. and 147 l of an aqueous NaOHsolution (300 g/l) added over 20 minutes with stirring. 2.16 kg of anucleus modifier in the form of a 57% aqueous glycolic acid solutionwere then added to the grey-blue suspension formed in order to reducethe particle size of the nuclei, and oxidation was performed for 7 hourswith 38 l of air per hour and per mol of iron.

[0084] The dark brown suspension was filtered out through a filter pressand the solid washed until the residual filtrate conductivity was 1mS/cm. The filter cake was dried at 70° C. in a circulating air dryingoven to a residual moisture of 5%, and the very hard blackish brown dryproduct was roughly ground in a roller crusher to particle sizes of upto 2 mm. The fine fraction <0.2 mm was separated out using a screen.

[0085] An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 20 nm, theneedle lengths between 50 and 80 nm. The particles were extremelyagglomerated. The BET specific surface area was 202 m²/g. The granulesthus obtained were placed directly in an adsorber tank with no furthertreatment.

[0086] The granules displayed an excellent adsorption performance inrespect of the contaminants contained in the flowing water anddemonstrated a high abrasion resistance, particularly when the adsorbertank is being back-flushed causing the granules to be whirled upstrongly. The abrasion value after 30 minutes was only 1%.

[0087] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.4 mg (As³⁺)/l was 1.0 mg(As³⁺)/g(FeOOH).h,for Na₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was2.07 mg(As³⁺)/g(FeOOH).h.

Example 3

[0088] 1.3 l of an aqueous 300 g/l NaOH solution were added to anα-FeOOH suspension obtained according to example 2 after a two-hourmaturation at 30° C. with stirring, and post-oxidation was performedsimultaneously for one hour with 190 l of air. The product was processedas described in example 2. Fine-particle needles of pure α-FeOOH with aBET specific surface area of 130 m²/g were obtained. Using a scanningelectron micrograph e.g. at a magnification of 60000:1, the needlewidths were measured at between 15 and 20 nm, the needle lengths between50 and 90 nm. The needles were extremely agglomerated. The granulesproved to be very mechanically and hydraulically resistant, and theabrasion value was only 3.9%.

[0089] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.3 mg/l was 1.1 mg(As³⁺)/g(FeOOH).h, forNa₂HAsO₄ with an original concentration of 2.8 mg/l it was 1.7mg(As³⁺)/g(FeOOH).h.

Example 4

[0090] 306 l of an aqueous NaOH solution (45 g/l) were prepared at 31°C. and 43 l of an aqueous solution of FeCl₂ (344 g/l) quickly added withstirring, and oxidation was then performed with 60 l of air per hour andper mol Fe. The dark yellow suspension thus obtained was processed inthe same way as in example 1.

[0091] An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 50 nm, theneedle lengths between 100 and 200 nm. The needles were extremelyagglomerated. The BET specific surface area was 132 m²l/g.

[0092] The granules thus obtained were placed in an adsorber tank withno further treatment. The granules displayed an excellent adsorptionperformance in respect of the contaminants contained in the water anddemonstrated a high abrasion resistance, particularly when the adsorbertank is being back-flushed causing the granules to be whirled upstrongly. The abrasion value after 30 minutes was only 12 wt. %.

[0093] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.4 mg (As³⁺)/l was 2.11 mg(As³⁺)/g(FeOOH).h,for Na₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was2.03 mg(As⁵⁺)/g(FeOOH).h.

Example 5

[0094] 124 l of an aqueous NaOH solution (114 g/l) were prepared at 24°C. and 171 l of an aqueous solution of FeSO₄ (100 g/l) quickly addedwith stirring, and oxidation was then performed with 10 l air per hourand per mol Fe. Immediately upon completion of oxidation, 56 l of anaqueous solution of Fe₂(SO₄)₃ (100 g/l) were added and stirred for 30minutes. The yellowish brown suspension thus obtained was processed inthe same way as in example 1.

[0095] An X-ray diffractogram showed that the product consisted of 100%α-FeOOH. Using a scanning electron micrograph e.g. at a magnification of60000:1, the needle widths were measured at between 15 and 35 nm, theneedle lengths between 70 and 180 nm. The needles were extremelyagglomerated. The BET specific surface area was 131 m²/g. The abrasionvalue after 30 minutes was only 7 wt. %.

[0096] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.3 mg (As³⁺)/l was 1.7 mg(As³⁺)/g(FeOOH).h,for Na₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was1.2 mg(As⁵⁺)/g(FeOOH).h.

Example 6

[0097] 7905 kg FeSO₄ were prepared, dissolved with water to a volume of53.3 m³, the solution cooled to 14° C. and 1000 kg MgSO₄.7 H₂O added tothis solution. The prepared solution was then diluted at 14° C. with5056 kg NaOH as a solution with approx. 300 g/l and then oxidised with4000 m³/h air to a precipitation degree of >99.5%. The batch was washedon a filter press until the residual filtrate conductivity was <1000μS/cm and the paste pushed through a perforated metal sheet with holediameters of 7 mm, causing it to be formed into strands. The strandswere dried on a belt dryer to a residual moisture of approx. 3%. AnX-ray diffractogram showed that the product consisted of 100% α-FeOOHwith very short needles. Using a scanning electron micrograph e.g. at amagnification of 60000:1, the needle widths were measured at between 30and 50 nm. The needle lengths could not be clearly determined as theneedles were too greatly agglomerated. The BET specific surface area was145 m²/g. The abrasion value after 30 minutes was only 6%.

[0098] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.5 mg (As³⁺)/l was 1.8 mg(As³⁺)/g(FeOOH).h,for Na₂HAsO₄ with an original concentration of 2.9 mg (As⁵⁺)/l it was1.5 mg(As⁵⁺)/g(FeOOH).h.

Example 7

[0099] 4096 kg NaOH (as solution with approx. 300 g/l) were prepared anddiluted with water to 40 m³. 4950 kg FeSO₄ were dissolved with water toform 48.5 m³ solution, cooled to 15° C. and then pumped into theprepared NaOH over 1 h. The suspension was then oxidised with 1500 m³/hair over approx. 2 h. Approx. 2 m³ of the nucleus suspension was washedon a filter press to obtain a filtrate conductivity <1000 μS/cm, thefilter cake was dried in a drying oven at 75° C. and the dried materialroughly ground to particle sizes <1.5 mm. The fine fraction <0.5 mm wasseparated out using a screen. The material thus obtained had a BETspecific surface area of 153 m²/g and consisted of 100% α-FeOOH. Using ascanning electron micrograph e.g. at a magnification of 60000:1, theneedle widths were measured at between 15 and 35 nm, the needle lengthsbetween 50 and 100 nm. The needles were extremely agglomerated.

[0100] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.7 mg (As³⁺)/l was 1.7 mg(As³⁺)/g(FeOOH).h,for Na₂HAsO₄ with an original concentration of 2.8 mg (As⁵⁺)/l it was1.4 mg(As⁵⁺)/g(FeOOH).h.

Example 8

[0101] 3100 kg NaOH (as solution with approx. 100 g/l) were measured outand diluted with cold water to 31 m³. The temperature of the solutionwas 26° C. 3800 kg FeSO₄ were dissolved with water to form about 38 m³solution, cooled to 13-14° C. and then pumped with stirring into theprepared NaOH. The suspension was then oxidised with 2500 m³/h air inapprox. 75 min. 18.2 m³ FeSO₄ solution (100 g/l) were added at a rate of150 l/min to this suspension with stirring and gassing. The suspensionwas filtered on a filter press and washed until the residual filtrateconductivity was <1000 μS/cm, the paste was pushed through a perforatedmetal plate and were dried on a belt dryer to a residual moisture ofless than 20%. The dry pellets were roughly ground to obtain a particlesize of less than 2 mm. The portion of the particles with less then 0.5mm was removed. The material thus obtained had a BET specific surfacearea of 145 m²/g and consisted of 100% α-FeOOH.

Example 9

[0102] An aqueous solution of FeSO₄ (100 g/l) was added at roomtemperature to 1600 g of the alkaline nucleus suspension preparedaccording to example 7 (2.7% FeOOH) with stirring and simultaneousaeration with 130 l/h of air until a pH of 8 was obtained. The nucleussuspension obtained was filtered, washed and the filter cake dried at75° C. and roughly ground to particle sizes of between 0.5 and 2 mm asdescribed in example 7. The material thus obtained had a BET specificsurface area of 163 m²/g and according to the X-ray diffractogramconsisted of 100% α-FeOOH. The scanning electron micrograph, e.g. at amagnification of 60000:1, showed that the needles are extremelyagglomerated. Adsorption performance: The adsorption rate for NaAsO₂with an original concentration of 2.7 mg (As³⁺)/l was 2.0mg(As³⁺)/g(FeOOH).h, for Na₂HAsO₄ with an original concentration of 2.7mg (As⁵⁺)/l it was 1.9 mg(As⁵⁺)/g(FeOOH).h, for KSb(OH)₆ (originalconcentration 3.0 mg (Sb⁵⁺)/l) the adsorption was 2.5 mg (Sb⁵⁺)/g(FeOOH).h, for Na₂CrO₄ (original concentration 47 μg (Cr⁶⁺)/l) 42 μg(Cr⁶⁺)/g(FeOOH).h were adsorbed, for PbCl₂ (original concentration 0.94mg (Pb²⁺)/l) 0.46 mg (Pb²⁺)/g(FeOOH).h were adsorbed.

Example 10

[0103] 6.4 l of an aqueous solution of NaOH (100 g/l) were prepared at29° C. with stirring and 12.2 l of an aqueous iron(II) sulfate solution(100 g/l) were added with simultaneous introduction of air until a pH of9 was obtained. The suspension thus obtained was processed in the sameway as in example 1. The material had a BET specific surface area of 251m²/g and according to the X-ray diffractogram consisted of 100% α-FeOOH.The scanning electron micrograph shows short, stumpy needles, which areextremely agglomerated. Abrasion performance: 5%.

[0104] Adsorption performance: The adsorption rate for NaAsO₂ with anoriginal concentration of 2.7 mg (As³⁺)/l was 1.1 mg(As³⁺)/g(FeOOH).h,for Na₂HAsO₄ with an original concentration of 2.7 mg (As⁵⁺)/l it was1.0 mg(As⁵⁺)/g(FeOOH).h.

[0105] Although the invention has been described in detail in theforegoing for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be limited by the claims.

What is claimed is:
 1. A unit suitable for the through-flow of a fluidmedium, at least partially filled with particles agglomerated fromfine-particle iron oxide and/or iron oxyhydroxide, wherein thefine-particle iron oxide and/or iron oxyhydroxide displays a particlesize of up to 500 nm and a BET surface area of 50 to 500 m²/g.
 2. Theunit of claim 1 wherein the medium comprises a gas.
 3. The unit of claim1 wherein the medium comprises a liquid.
 4. The unit of claim 3, whereinthe liquid comprises water.
 5. A water treatment plant comprising theunit of claim
 4. 6. A waterwork comprising the water treatment plant ofclaim
 5. 7. The unit of claim 1 wherein the fine-particle iron oxideand/or iron oxyhydroxide displays a BET surface area of 80 to 200 m²/g.8. The unit of claim 1 wherein the agglomerates further comprise ironoxide pigments with particle sizes above 500 nm and BET surface areasbelow 50 m²/g, whereby the maximum content of these is such that theresistance of the charge to the forces exerted upon it by the flowingmedium is sufficiently great that the stress exerted on the charge bythe flowing medium does not lead to an undesirable abrasion of thecharge material.
 9. A process for the production of particles fromfine-particle iron oxide and/or iron oxyhydroxide comprising the stepsof producing an aqueous suspension of fine-particle iron oxides and/oriron oxyhydroxides having a BET surface area of 50 to 500 m²/g, andremoving the water and dissolved constituents by either I) a) firstremoving only the water from the suspension, b) introducing the residuethus obtained in water, c) filtering the material obtained, d) washingthe residue, and e) either e1) completely dehydrating the filter cakeobtained as residue and comminuting the material thus obtained to thedesired shape and/or size or e2) partially dehydrating the filtercake toobtain a paste, shaping the paste and subsequently additionally dryingthe paste until a pellet is obtained, or II) a) filtering thesuspension, b) washing the residue, c) either c1) completely dehydratingthe filter cake obtained as residue in the form of a solid to semisolidpaste and then comminuting the material thus obtained to the desiredshape and/or size or c2) partially dehydrating the filtercake to obtaina paste, shaping the paste, and subsequently additionally drying thepaste until a pellet is obtained.
 10. The process of claim 9 furthercomprising a step of subjecting the pellet to a further comminution bygrinding or rough grinding.
 11. The process of claim 9 wherein the BETsurface area is 80 to 200 m²/g.
 12. The process of claim 9 wherein I) a)the water is removed by evaporation.
 13. The process of claim 9comprising I d) washing the residue until it is free from salts.
 14. Theprocess of claim 9 comprising II b) washing the residue until it is lowin salts.
 15. A process comprising treating fluid medium by contactingthe gas or liquid in a unit according to claim 1 with particles obtainedby a process for the production of particles from fine-particle ironoxide and/or iron oxyhydroxide comprising the steps of producing anaqueous suspension of fine-particle iron oxides and/or ironoxyhydroxides having a BET surface area of 50 to 500 m²/g, and removingthe water and dissolved constituents by either I) a) first removing onlythe water from the suspension, b) introducing the residue thus obtainedin water, c) filtering the material obtained, d) washing the residue,and e) either e1) completely dehydrating the filter cake obtained asresidue and comminuting the material thus obtained to the desired shapeand/or size or e2) partially dehydrating the filtercake to obtain apaste, shaping the paste and subsequently additionally drying the pasteuntil a pellet is obtained, or II) a) filtering the suspension, b)washing the residue, c) either c1) completely dehydrating the filtercake obtained as residue in the form of a solid to semisolid paste andthen comminuting the material thus obtained to the desired shape and/orsize or c2) partially dehydrating the filtercake to obtain a paste,shaping the paste, and subsequently additionally drying the paste untila pellet is obtained.
 16. The process of claim 15 wherein the fluidmedium comprises water.
 17. The process of claim 15 comprising removinga heavy metal, phosphorus, antimony, beryllium, selenium, tellurium orcyano compound from water.
 18. The process of claim 15 comprisingremoving arsenic compounds from water.